Kinetics of the photo-addition of HBr to ethylene at low pressures

Abstract

The kinetics of the photo-initiated addition of HBr to ethylene in the gas phase, at pressures of 25–125 torr, have been investigated. Using initial rates (< 5 % reaction) the kinetic expression is rate (initial)=kI₀[ethylene][HBr]²(total pressure),^0.4 which can be rationalized in terms of the conventional chain mechanism. However, as ethyl bromide, the reaction product, accumulates, or if it is added to the reaction mixture, the kinetic expression changes to, rate =k′I^½₀[ethylene][HBr][ethyl bromide]^½. These changes are rationalized in terms of the following four assumptions: (i) The addition of a bromine radical to an ethylene molecule produces a vibrationally excited bromoethyl radical, which has within itself the energy needed for its decomposition. (ii) This excited radical can be efficiently deactivated to energies closer to thermal by ethyl bromide, or any gaseous alkyl bromide with the exception of n-propyl bromide. (iii) Ethyl bromide is an exceptionally efficient third-body for the recombination of bromine radicals (iv) The wall recombination of bromine radicals requires an activation energy of about 5–6 kcal/mole.